The embodiments described herein relate to the exposure of X-rays, and in particular, to a method for controlling the X-ray exposure in a CT system during a scanning process.
An X-ray CT system emits X-rays to a subject (patient), detects by a detector the X-ray absorption coefficient in such human body tissues as organs, blood and gray matter, and processes (reconstructs) said absorption coefficient by a computer to provide an image (tomographic image) of the sectional plane (slice plane) of the area to be examined.
A doctor diagnoses the patient's condition on the basis of the tomographic image of a predetermined area to be examined that is reconstructed by the X-ray CT system. For this purpose, the image quality of the reconstructed tomographic image must fine enough to distinguish the difference of the X-ray absorption coefficients of the human tissues and to match with the purpose of the examination. To obtain such image quality, the image noise shall be reduced, i.e. to reduce the image noise and obtain high quality tomographic image, the amount of X-rays transmitted by the subject and detected on the detector must be large. As a result, in order to obtain enough transmitted X-rays on the detector, the amount of X-rays emitted to the subject must be large.
However, increasing the amount of X-rays emitted to the subject to improve the image quality will result in a undesired increase in the exposure of the patient to X-ray, so in actual practice, real-time control is required so that the amount of X-rays emitted from the X-ray tube is the minimum amount necessary to get the required image quality.
The amount of X-rays emitted from the X-ray tube is controlled by the current transferred to the X-ray tube (hereinafter referred to as tube current). Conventional X-ray CT system usually provides a function of controlling tube current to achieve said minimum X-ray emission (including automatic tube current control function). As shown in FIG. 1, the automatic tube current control function in such conventional X-ray CT system includes the following steps: step 100, performing a scout scan on a predetermined area of the subject in a predetermined direction to obtain desired data of the scout scan; step 101, analyzing and processing the data obtained in step 100 so as to calculate the eccentricity and the value of the projection area of an elliptic slice of the area to be examined at the scout scan position (the Projection Measure reflects the length of the long axis or short axis of the elliptic slice, the eccentricity can be derived from the projection area value and the Projection Measure), said slice being almost an ellipse; step 102, calculating the tube current for exposure on the basis of the Projection Measure, projection area value, noise desired by the doctor as well as the intrinsic parameters of the system. The system performs exposure and axial scan on the slice based on the calculated tube current. In other words, the tube current mA in automatic tube current control is a function of the projection area Pa and Projection Measure as the following: mA=f (Pa, Projection Measure, desired image noise value, system parameters). Then it can be appreciated that, when the X-ray CT system is determined, the tube current is mainly determined by the Projection Measure obtained from scout scan and the projection area value of the slice, so only accurately calculated Projection Measure and projection area value of the slice can yield tube current that really matches the slice to be scanned.
However, in actual application, the center of the subject being scanned moves up and down with the scan bed, thus deviating from the rotation center of the scan device of the X-ray CT system, and so the data obtained from scout scan is not accurate, resulting in a discrepancy between the calculated projection area value of the slice and the real projection area value of the slice. The final result is that the calculated tube current is either too large or too small, causing the patient under examination either to receive too much X-rays and get hurt or to receive too little and leave the doctor unable to accurately determine the state of an illness. For example, as shown in FIG. 3, a patient lies on a bed deviating from the rotating center ISO of the CT system, and 0 degree and 180 degree scout scans are carried out, with the doctor still using said automatic tube current control function to scan the patient. Even if in the same scan position, the tube current value and the image noise value of axial scan results may differ by a large extent. Clinical practice often has such results: the tube current value of axial scan after 180 degree scout scan is twice that after 0 degree scout scan. Too large tube current value will bring extra harm to the patient. Such tube current control function may produce contradictory results, making it difficult for the doctor to promote the clinical application of the tube current control function.